Nicer alpha expansion algorithm

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Bottersnike 2023-02-17 21:56:58 +00:00
parent 697eeeb99e
commit f0ee40b08c

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@ -41,9 +41,10 @@
key is always 32768 (8000<sub>h</sub>) bytes, and is unmodified during this process.
</p>
<p>
Keys are derrived based on a key expansion algorithm that takes as input an unsigned short (16 bit) seed. I'm not
totally sure what expansion algorithm this is, or if it's something totally custom, but for now here's a snippet of
python code that implements the expansion:
Keys are derrived based on a key expansion algorithm that takes as input an unsigned short (16 bit) seed. This
algorithm is fundamentally an LSFR-based PRNG, used to generate a stream of bytes, which becomes the key. The
<code>& 0xffffffff</code> is not strictly necessary but is convenient in python in order to avoid
<code>seed</code> blowing up in size.
</p>
<pre>
@ -51,26 +52,17 @@
def amAuthDiskInitKey(seed):
key = bytearray(0x8000)
for i in range(0x8000):
uVar1 = (seed * 2 >> 4 ^ seed * 2) >> 10 & 2 | seed << 2
seed <<= 1
for i in range(len(key)):
x = seed
uVar2 = uVar1 * 2
uVar3 = ((seed << 2) >> 4 ^ uVar1) >> 10 & 2 | uVar2
uVar1 = uVar3 * 2
uVar3 = (uVar2 >> 4 ^ uVar3) >> 10 & 2 | uVar1
for _ in range(8):
x |= ((((seed >> 4) ^ x) >> 11) & 1)
seed <<= 1
x <<= 1
key[i] = (x >> 1) & 0xff
uVar2 = uVar3 * 2
uVar3 = (uVar1 >> 4 ^ uVar3) >> 10 & 2 | uVar2
uVar1 = uVar3 * 2
uVar3 = (uVar2 >> 4 ^ uVar3) >> 10 & 2 | uVar1
uVar2 = uVar3 * 2
uVar3 = (uVar1 >> 4 ^ uVar3) >> 10 & 2 | uVar2
uVar1 = uVar3 * 2
uVar2 = (uVar2 >> 4 ^ uVar3) >> 10 & 2 | uVar1
seed = uVar2 | (uVar1 >> 4 ^ uVar2) >> 11 & 1
key[i] = seed & 0xff
seed = x & 0xffffffff
return key
{% endhighlight %}</pre